Theoretical astrophysics and plasma physics at RPC

Abstract:

The frequency of compact object interactions in AGN discs is naturally tied to the number of objects embedded within it. We investigate the evolution of black holes in the nuclear stellar cluster on inclined orbits to the AGN disc by performing adiabatic hydrodynamical simulations of isolated black hole disc crossings over a range of disc densities and inclinations . We find radiation dominates the pressure in the wake that forms around the BH across the full inclination and disc density range. We identify no well defined steady state wake morphology due to the thin geometry of the disc and the vertical exponential density drop off, where the wake morphology depends on the vertical depth of the transit within the disc. The inclination damping relative the pre-transit inclination behaves as a power law in and the ambient Hill mass as . The drag on the BH is dominated by the gravity of the wake for the majority of our inclination range until accretion effects become comparable at , where is the disc aspect ratio. At low inclinations () the wake morphology becomes more spherical, leading to a regime change in the inclination damping behaviour. Our results suggest that the inclination damping time-scale is shorter than expected from only episodic Bondi–Hoyle–Lyttelton accretion events during each transit, implying inclined objects may be captured by the AGN disc earlier in its lifetime than previously thought.

Abstract:

We investigate close encounters between initially unbound black holes (BHs) in the gaseous discs of active galactic nuclei (AGNs), performing the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary BH formation. We discuss a suite of 135 simulations, considering nine AGN disc environments and 15 BH impact parameters. We find that the gas distribution within the Hill sphere about an isolated embedded BH is akin to a spherically symmetric star with a low-mass convective envelope and a BH core, with large convective currents driving strong outflows away from the mid-plane. We find that Coriolis force acting on the outflow results in winds, analogous to cyclones, that counter-rotate with respect to the mid-plane flow within the Hill sphere. We confirm the existence of strong thermal blasts due to minidisc collisions during BH close encounters, as predicted in our previous 2D studies. We document binary formation across a wide range of environments, finding formation likelihood is increased when the gas mass in the Hill sphere is large, allowing for easier binary formation in the outer AGN disc. We provide a comprehensive overview of the supermassive black hole’s role in binary formation, investigating how binary formation in intermediate density environments is biased towards certain binary orientations. We offer two models for predicting dissipation by gas during close encounters, as a function of the ambient Hill mass alone, or with the periapsis depth. We use these models to motivate a prescription for binary formation likelihood that can be readily applied to Monte Carlo simulations of AGN evolution.

Abstract:

The ITER Neutral Beam Injection (NBI) system is based on negative ions, produced in an RF-driven plasma source. The ITER NBI lines must deliver a current density of 230 A/m2 of negative hydrogen ions, accelerated to 870 keV, or a current density of 200 A/m2 of negative deuterium ions accelerated to 1 MeV. NBI systems, based on negative ions, are compromised by a process known as stripping losses, in which negative ions are neutralized in the grid system before achieving full energy. For a source filling pressure of p fill = 0.3 Pa, 29% of the extracted H -(D -) ions are predicted to be lost by stripping in the ITER full-scale NBIs system (7 grid acceleration system). To compensate for these stripping losses, a larger amount of negative ions has to be extracted from the source (329 A/m2 in hydrogen and 286 A/m2 in deuterium). The ELISE test facility is based on a 1/2-size ITER source. It extracts H -(D -) ions using a 3-grid acceleration system, with a maximum extraction voltage of 10 kV and acceleration voltage of 50 kV is achieved. In a 3-grid acceleration system, 10% of stripping losses is predicted for both isotopes. This contribution focuses on experimental measurements of stripping losses at ELISE. Experimentally, stripping losses are monitored using Beam Emission Spectroscopy (BES), which analyzes the Doppler-shifted spectrum of the Balmer Hα (Dα ). To not underestimate the number of stripping losses the full area between the unshifted Peak background (H 2 dissociation and excitation) and the Doppler Peak (fully-accelerated beam particles excitation) needs to be considered. However, the influence of BES background and signal-to-noise ratio (SNR) can affect the calculation of stripping losses, mainly for hydrogen measurements at low filling pressures (< 0.4 Pa). To accurately predict the value of the stripping losses, only signals with high-enought SNR should be used. When this effect is considered, no differences between hydrogen and deuterium are found in terms of stripping losses. For a filling pressure of 0.3 Pa, a stripping fraction of 6.0±0.8% was found for hydrogen and 6.2±0.7% for deuterium. A systematic comparison of the stripping losses between hydrogen and deuterium under various experimental conditions is presented.